Research Article

Received: 4 May 2011 Revised: 2 August 2011 Accepted: 10 September 2011 Published online in Wiley Online Library: 27 October 2011

(wileyonlinelibrary.com) DOI 10.1002/jrs.3075

Doping effect on the phase transitiontemperature in ferroelectric SrBi2�xNdxNb2O9

layer-structured ceramics: a micro-Ramanscattering studyKai Jiang, Wenwu Li, Xiangui Chen, Zhenni Zhan, Lin Sun, Zhigao Hu*and Junhao Chu

The temperature dependence of Raman spectra for SrBi2�xNdxNb2O9 ceramics (x from 0 to 0.2) has been studied in a wide tem-perature range from 80 to 873K. It is found that the peak position of the A1g[Nb] phonon mode at 207 cm–1, which is directlyassociated with the distortion of NbO6 octahedron, decreases with increasing Nd composition, while the A1g[O] phonon modeat 835 cm–1 increases. Moreover, both the peak position and intensity of the A1g[Nb] phonon mode reveal strong anomaliesaround the ferroelectric to paraelectric phase transition temperature. It indicates that the phase transition temperaturedecreases from about 710 to 550K with increasing Nd composition, which is due to the fact that the introduction of Nd ionsin the Bi2O2 layers reduces the distortion extent of NbO6 octahedron. Copyright © 2011 John Wiley & Sons, Ltd.

Keywords: SrBi2�xNdxNb2O9 ceramic; phase transition; Raman spectroscopy

* Correspondence to: Zhigao Hu, Key Laboratory of Polar Materials and Devices,Ministry of Education, Department of Electronic Engineering, East ChinaNormal University, Shanghai 200241, China E-mail: zghu@ee.ecnu.edu.cn

Key Laboratory of Polar Materials and Devices, Ministry of Education Departmentof Electronic Engineering, East China Normal University Shanghai 200241, China

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Introduction

Bismuth layer structure ferroelectrics (BLSFs) have attracted con-siderable attention because of their potential application inferroelectric random access memories, electro-optic switches,and other electronic devices.[1–3] The general formula of BLSFsis given as (Bi2O2)

2+(Am�1BmO3m+1)2–, where A and B are the

two types of cations that enter the perovskite unit, and m isthe number of perovskite unit cell between bismuth oxidelayers.[3–5] Especially, strontium barium niobate SrBi2Nb2O9

(SBN), which is known to be anm= 2 member of the BLSFs family,has been regarded as a promising ferroelectric material becauseof low dielectric constants and excellent fatigue resistance.[5,6]

It is well known that SBN crystallizes in the orthorhombic spacegroup A21am at room temperature (RT). At the temperatureabove the ferroelectric Curie temperature (TC� 705 K), the struc-ture of SBN belongs to the tetragonal paraelectric phase. Thestructure corresponds to I4/mmm symmetry, which can result in12 Raman-active modes: 4A1g+2B1g+6Eg.

[7,8]

Recently, many research groups have focused on the dopingeffect on the fabrications and dielectric properties for SBN mate-rials.[2,3,9–12] The substitution of Ca ions in the Sr site for the SBNceramic induced the increase in TC, which is useful for the appli-cation in high-temperature resonators.[11,12] However, the substi-tution of some rare earth ions such as La3+ or Pr3+ for Bi3+ in theBi2O2 layers can result in a shift for the TC to lower tempera-ture.[3,9,13] It is found that the behavior of the Nd-doped SBNceramic tends to change from a normal ferroelectric to a relaxortype ferroelectric because of the introduction of Nd ions in theBi2O2 layers.

[3,9,10,14] Up to now, a detailed understanding of thelattice dynamic properties and the phase transition behavior ofNd-doped SBN ceramics is still lacking. Raman spectroscopy is a

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sensitive technique for investigating the structure modificationsand lattice vibration modes, which can give the information onthe changes of lattice vibrations and the occupying positions ofdoping ions. Thus, it is a powerful tool for the detection of phasetransition in the doping-related ferroelectric materials.[14–16]

In this paper, we have investigated the Nd doping effect on theRaman phonon modes and the phase transition from ferroelectricto paraelectric of SrBi2�xNdxNb2O9 (SBNN) ceramics by tempera-ture-dependent Raman scattering. The Nd composition dependenceof the phase transition temperature has been discussed in detail.

Experimental

The SBNN (x= 0, 0.05, 0.1, and 0.2) ceramics were prepared by aconventional solid-state reaction route, and SrCO3, Bi2O3,Nb2O5, and Nd2O3 were used as the starting materials. Detailsof the fabrication process for the ceramics can be found else-where.[6,10] The X-ray diffraction analysis at RT indicates that theSBNN ceramics are of orthorhombic phase.[6] Raman scatteringexperiments were carried out using a Jobin–Yvon LabRAM HR800 UV micro-Raman spectrometer, excited by a 632.8 nmHe–Ne laser with a spectral resolution of 0.65 cm–1. Temperature-dependent measurements from 80 to 873K were performed usingthe Linkam THMSE 600 heating stage, and the set-point stability isof better than 0.5K.

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Results and Discussion

Figure 1(a) shows the Raman spectra of the SBNN ceramics withdifferent Nd compositions at 80 K in the spectral range of50–950 cm–1. The scatter circle points are the experimental dataand the solid lines are the fitting curves according to theLorentzian multipeak fitting. The Raman selection rules allow 18phonon modes (4A1g+2B1g+6B2g+6B3g) for the SBN ceramicsat low temperature.[8] However, less than 12 phonon modes areobserved because of the possible overlapping of the same sym-metry vibration or the weak feature of some vibration bands.[17]

According to the assignment of SrBi2Nb2O9 single crystal,[8] theRaman phonon modes at about 61, 207, and 835 cm–1 can beassigned to the A1g phonon mode, the vibrations at about 179and 579 cm–1 can be assigned to the Eg phonon mode. However,the assignments of other phonon modes are still not clear.The internal vibrations of NbO6 octahedra occur in thehigh-wavenumber mode region above 200 cm–1 because theintragroup binding energy within the NbO6 octahedra is muchlarger than the intergroup or crystal binding energy.[11] The com-position dependence of the Raman shift for two typical phononmodes is illustrated in Fig. 1(b). Note that the modes at 207 and835 cm–1 do not shift in the same direction with increasing Nbsubstitution. The A1g[Nb] phonon mode at 207 cm–1, which arisesfrom the distortion of NbO6 octahedra, generally decreases withthe Nd composition. It indicates that the introduction of Nd ionsin the Bi2O2 layers may reduce the degree of distortion of NbO6

octahedra.[10] However, the A1g[O] phonon mode at 835 cm–1

mode corresponding to the symmetric Nb�O stretching vibra-tion, increases with the substitution of Nd ions for Bi ions.Because the Nd3+ ions incorporation into Bi2O2 layers inducesthe bond relaxation in this layer, the neighboring NbO6 octahe-dra might shrink, resulting in a blueshift in the peak position ofthe A1g[O] phonon mode.[2]

To further understand the influence of Nd3+ ion substitution inthe Bi2O2 layers on the phase transition behavior for the SBNNceramics, we present in Figs 2 and 3 the temperature depen-dence of the Raman spectra for all the SBNN ceramics in the

Figure 1. Raman scattering spectra of the SBNN ceramics with differentNd composition (x) recorded at 80 K. The scatter circle points are theexperimental data and the solid lines are the fitting curves. The dashedlines clearly indicate some Raman-active phonon modes. The inset showsthe peak wavenumber variation of the A1g[Nb] phonon mode at about207 cm–1 and the A1g[O] phonon mode at about 835 cm–1 as a functionof the Nd composition.

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temperature range from 80 to 873 K. It is found that the A1g[O]phonon mode at about 835 cm–1 becomes broad and asymmet-ric, and there is no remarkable shift of this peak with the temper-ature. A strong broadening peak at about 579 cm–1 can beobserved at 80 K because of the combined effects of two modessplitting from the Eg character mode. The broadening band canbe assigned to a rigid sublattice mode, in which all the positiveand negative ion displacements are equal and opposite.[7] Withincreasing temperature, the peak position and intensity of thephonon mode present a decreasing trend. Because the mode isassigned to the asymmetric Nb�O vibration, it can be concludedthat the NbO6 octahedra is sensitive to the temperature. How-ever, the A1g[Bi] phonon mode at about 60 cm–1 remainsunchanged with increasing temperature, which indicates thatthe vibration of Bi ions in the Bi2O2 layers are not sensitive tothe temperature. On the other band, the phonon modes at about280 and 310 cm–1 (labeled by *), which are associated with theO�Nb�O bending, become more difficult to be distinguishedas the temperature increases and disappear at high temperature.Similar phenomena can be observed for the phonon modes inthe range of 430–455 cm–1 (labeled by #). The band at about455 cm–1, which is ascribed to a Nb�O torsional mode, has beenassigned as the Eg character and splits into two phonon modescentered at 430 and 455 cm–1 at lower temperature. As pointedout by Graves et al.,[8] it can be ascribed to the fact that the sev-eral Eg phonon modes split into the B2g and B3g phonon modesduring the tetragonal to orthorhombic transition. Moreover, thesplitting of the phonon modes reveals the structural changes inthe SBNN ceramics with the temperature.

In general, the phase transition in the BLSFs cannot be deter-mined in terms of a single soft mode. They can be driven byvarious combinations for the displacive modes.[18] It is clear thatthe Eg[Nb] phonon mode at 179 cm–1 have a remarkable redshiftwith increasing temperature. A similar behavior of shifting to lowwavenumber is also observed for the phonon mode at 97 cm–1 ofthe SBNN ceramics. In addition, the Eg[Nb] and A1g[Nb] phononmodes are gradually overlapped with increasing temperaturebecause of the disordered structure for the SBNN ceramics. Thetemperature dependence of the peak positions for the threeRaman phonon modes (the Eg[Nb], A1g[Nb] phonon modes andthe mode at 97 cm–1) is plotted in Fig. 4. The wavenumber ofthe Eg[Nb] phonon mode decreases from 179 to 167 cm–1 whenthe temperature is changed from 80 to 873 K. The phonon modeat 97 cm–1 is shifted toward a lower wavenumber side of 8 cm–1.The observed shifting of the peak position with the temperaturecan be ascribed to the effect of the thermal broadening and dis-order: the anharmonic effects of the lattice. On the other hand,the change in bond length between the oxygen and cationscan also induce the shifting.[17] Note that an interesting temper-ature dependence of the wavenumber and intensity of the A1g[Nb] phonon mode was observed in this temperature region.The phonon mode originates from the distortion of vibrationNbO6 octahedra, which could reflect the distortion degree ofNbO6 octahedra. Thus, it can be expected that the A1g[Nb] pho-non mode should be appropriate to probe the ferroelectric toparaelectric phase transition for the SBNN ceramics.

For all the SBNN ceramics, the peak position of the A1g[Nb]phonon mode decreases as the temperature is increased. How-ever, an obviously anomalous behavior occurs beyond the phasetransition temperatures: the wavenumber of this phonon modesharply increases with increasing the temperature. Note thatthe temperature of the anomalous points is different for the four

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Figure 2. Temperature dependence of the Raman spectra for SBNN ceramics with the composition of x=0 and 0.05. The dashed arrows show the shiftof the peak position for the phonon modes with the temperature. The symbol asterisk (*) and pound sign (#) indicate the two weak Eg phonon modes inthe range of 280–310 cm–1 and 430–455 cm–1, respectively.

Figure 3. Temperature dependence of the Raman spectra for SBNN ceramics with the composition of x=0.1 and 0.2. Note that the symbols and arrowshave the same meaning as those in Fig. 2.

Doping effect on the phase transition temperature in ferroelectric SrBi2�xNdxNb2O9 ceramics

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ceramics: 710 K for x= 0, 665 K for x= 0.05, 625 K for x= 0, 1 and550 K for x= 0.2. The different anomalous points can be ascribedto the different Nd3+ incorporation in the Bi2O2 layers. The anom-alous change in the A1g[Nb] phonon mode suggests that thephase transition from the orthorhombic to the tetragonal phaseoccurs at the temperature. The dashed lines in Fig. 4 shows theTC for the four ceramics samples. It also indicates that the intro-duction of Nd leads to a decrease in the transition temperature.

Similar phenomena can be experimentally observed in thevicinity of the TC when following the temperature evolution ofthe intensities for the three phonon modes in Figs 4(b), (d), (f),and (h). Generally, the intensities of the A1g[Nb] phonon modefor all the ceramics increase with the temperature. However, asharp increase occurs as the temperature passes the TC, whichalso indicates that there is the structural change in the SBNN

J. Raman Spectrosc. 2012, 43, 583–587 Copyright © 2011 John

ceramics. The same behavior can be found in the intensities forthe Eg[Nb] phonon mode and the mode at 97 cm–1. It is impor-tant to point out that the anomalous trends have been alsoreported for the ferroelectric to paraelectric phase transition ofother ferroelectric materials (such as Bi4�xNdxTi3O12, LiCsSO4

crystal, and BaTiO3 nanotube arrays, etc.) using Raman scatter-ing.[17,19,20] A sharp change in the temperature dependence ofboth wavenumber and intensity of the A1g[Nb] phonon modewas applied to probe the phase transition of the SBNN ceramics.

The transition temperatures as a function of the Nd composi-tion obtained from the Raman analysis are presented in Fig. 5.The TC for the SBNN ceramics shifts to lower temperature withincreasing Nd composition, and it can be well expressed by(707–794.3x) K with the solid line. The result demonstrates thatthe TC decreases from about 710 to 550 K as the Nd composition

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Figure 4. Raman shift and intensities of several Raman phonon modes (the A1g[Nb], A1g[O] and 97 cm–1 mode) as a function of the temperature for theSBNN ceramics. The dashed lines indicate that the anomalous behavior occurs in the vicinity of the ferroelectric to paraelectric phase transitiontemperatures.

Figure 5. The Nd composition dependence of phase transition temper-ature in the SBNN ceramics as determined from the anomalous behaviorsof several phonon modes in the Raman spectroscopy (★). For compari-son, the variations of the TC obtained by dielectric constant measure-ments for some rare earth ions doping in SBN ( , , ) are also given.The solid line is the linearly fitting result to guide the eyes.

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increases from 0 to 0.2. It indicates that the substitution of Ndions for Bi3+ ions in the Bi2O2 layers results in the linear reductionof the TC. The present work is in good agreement with the resultsfrom the dielectric spectroscopy.[10] Compared with the effect onthe transition temperature with the substitution of La3+ or Pr3+

for Bi3+ site, it can be concluded that the introduction of theserare ions in the Bi2O2 layers can induce a shift toward lower tem-perature for the TC. This can be attributed to the substitution ofthese ions (having no lone pair electrons) at the bismuth site(having one lone pair of 6 s2 electrons) resulting in the reductionin the distortion extent of NbO6 octahedron. According to the

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valence shell electron pair repulsion theory,[10] the lone pairelectrons tend to occupy more space than the bonding pair elec-trons. Thus, a significant redshift can be observed in the A1g[Nb]phonon mode of the SBNN ceramics with different Nd composi-tion. In addition, the tilting angle of the NbO6 octahedronobtained from Nb�O�Nb bond angle, which could reflectthe structure distortion degree of octahedron, decreases withincreasing Nd composition.[8] Furthermore, the tensile and com-pressive strain arising from the Bi2O2 layers and the perovskite-like layers in BLSFs, respectively, can also affect the structure ofthe NbO6 octahedron. As a result, the substitution of the isova-lent, larger, and nonpolarizable Nd3+ ions for the Bi3+ ions inBi2O2 layers is expected to affect the bonding and hence theforce constants among various ions. Therefore, there is a pro-nounced influence on the distortion of the NbO6 octahedron,which results in the decrease of the transition temperature fromthe ferroelectric to the paraelectric state.

Conclusions

In summary, the composition dependence of phase transitiontemperature in the ferroelectric SBNN ceramics has beeninvestigated using Raman spectroscopy in the temperaturerange from 80 to 873 K. It was found that a ferroelectric toparaelectric phase transition temperature decreases from 710to 550 K with increasing Nd composition. The phase transitioncan be reflected by the critical changes in temperaturedependence of wavenumber and intensity of the A1g[Nb]phonon mode. The decrease of the TC can be attributed tothe following reason: the reduced distortion extent and tiltingangle of NbO6 octahedron because of the Nd3+ incorporationinto the Bi2O2 layers.

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Doping effect on the phase transition temperature in ferroelectric SrBi2�xNdxNb2O9 ceramics

Acknowledgements

This work was financially supported by Natural Science Founda-tion of China (Grant Nos. 60906046 and 11074076), Major StateBasic Research Development Program of China (Grant Nos.2007CB924901 and 2011CB922200), Program of New CenturyExcellent Talents, MOE (Grant No. NCET-08-0192) and PCSIRT,Projects of Science and Technology Commission of ShanghaiMunicipality (Grant Nos. 10DJ1400201, 10SG28, 10ZR1409800,and 09ZZ42), and The Program for Professor of Special Appoint-ment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

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